The coating of ceramic articles with glass ceramic materials has been a known industrial practice for a long time, and has already been described in previous patents, for example Italian Patent No. 913,836 to the applicant Marazzi.
To obtain a coating usable with ceramics, or to give rise to the crystallization of polycrystalline phases starting from a vitreous matrix, it is necessary that the composition be carefully studied so as to favor crystallization of the desired phases.
For example compositions rich in zinc have been used for a long time, to obtain coatings for ceramic articles. In these compositions the crystalline phases which are formed during devitrification are always made up of zinc silicates, for example Willemite.
Glass ceramic materials assimilable with this family are known, for example as described in U.S. Pat. Nos. 2,920,971, 3,460,987, 3,681,097, 3,681,102, 3,839,053, 3,854,963, 3,951,669, 4,199,340, 4,526,873, 4,687,750 and French Patent No. 1,496,496.